Multiplex telephony.



No. 857,079. PATBN'IED JUNE 18, 1907. M. LEBLANC.

MULTIPLEX TELEPHONY.

APPLICATION FILED maze. mos. RENEWED MAR. 19. 1907.

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PATENTED JUNE 18, 1907.

M. LEBLANG.

MULTIPLEX TELEPHONY.

APPLICATION FILED rmme. 190s. RENEWED MAR. 19, 1907.

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. IINITED STATES PATENT OFFICE.

MAURICE LEBLANC, OF PARIS, FRANCE, ASSIGN OR TO WESTINGHOUSE ELECTRIC AND MANUFACTURING COMPANY, A CORPORATION OF PENN- SYLVANIA.

MULTIPLEX TELEPHONY.

Specification of Letters Patent.

Patented June 18, 1907.

Application filed February 26, 1903- Renewed March 19, 1907. Serial No. 363,212.

citizen of the Republic of France, residing at Villa Montmorency, No 1 Avenue de Bonffleurs, Paris, France, have invented a new and useful Improvement in Telephony, of which the following is a specification.

My invention has reference to improvements in telephony and more especially in multiple telephony: and it comprises a new method of and apparatus for transmitting telephonic messages.

In order that my invention may be more readily understood, I begin by referring to the prior state ofthe art.

In Letters Patent Number 596,017 granted to I-Iutin & Leblanc on December 21st, 1897, there is described a means of transmitting a telephonic message over a single line wire which consists in maintaining upon the line an alternating current of a frequency which is so high as to be inaudible in the telephonic receiver and in modifying this alternating current by a telephonic transmitter, to superpose thereon waves of current corresponding to vocal sounds, so that the modified waves are now rendered audible in the telephonic receiver.

In an application for patent filed by Hutin & Leblanc on May 9th, 1894, under Number 510,658, there is described a means of sending a number of telephonic messages simultaneousl y over the same line wire and receiving each message in that telephonic receiver which is apportioned thereto, which means consists in superposing upon the line a number of alternating currents of high but differing frequencies, each of which alternating currents is sent into that receiving branch only at the receiving end of the line which is electrically tuned thereto by making its reactance zero for the periodicity of current which it is to receive, but which alternating currents are normally inaudible in the telephone receivers placed in these receiving branches by reason of the fact that the fre-- quencies of the alternating currents are too high for audibility. By modifying these alternating currents of normally inaudible frequency by telephonic transmitters at the transmitting end of the line in such a manner that each telephonic transmitter acts upon and modifies only that frequency of alternat ing currents which has been el'pnortioneddo it, there results that the sounds which are uttered before a given transmitter are received at a given receiver and that receiver only.

The method of multiplex telephony which has just been described primarily depends upon the properties of electrically tuned or electrically resonant circuits of zero reactanoe for a given frequencyof current. In

.Patent Number 628,246 granted July 4th,

1899, Hutin & Leblanc describe a method of 'inultiplex telephony which primarily depends upon a different principle. I-Iutin & Leblanc, in this patent, show that if a multiphase current of a given frequency is sent over the line and is modified in accordance with voice vibrations, it will be received and rendered audible in a telephone receiver which is excited by multiphase currents of the same frequency; but it will not be received and rendered audible if the difference between the fre uency of the modified multiphase current w ich comes over the line and of the multiphase current which excites the telephonic receiver attains a value above a certain minimum much smaller than the frequency of the lowest tones of the human voice. In this patent, therefore, Hutin & Leblanc, in order to have a system of multiplex telephony, send different frequencies of multiphase current over the line, each modified by a different telephone transmitter and each received and rendered audible inithat receiving telephone only at the receiving end of the line which is excited by multiphase currents of corresponding periodicity. But while the invention of Hutin & Leblanc covered by their Patent Number 628,246 comprehends the use of an alternating current irrespective of the number of its phases, the spe-- cific system which is shown in the atent necessitates the use of multiphase a ternating currents which require two or more line wires and a return circuit.

It is the object of my invention to devise a system for multiplex telephony in which no more than a single line wire and a return circuit is rendered necessary and in which single phase alternating currents are employed.

In the drawings Figure 1 shows a diagram of the circuits at one station. Fig. 2 shows a diagram of the circuits when three stations at each end of the line are used, and Fig. 3

shows a diagram of the circuits when three stations at each end of the line and central ICC . cites the microphone int-o a varying or an sounds of a pitch cc.

sources of alternating currents of different I frequencies are used.

In the systems of telephony now in commercial use, the microphone is excited by direct or continuous currents as distinguished from alternatin currents. That is to say, a direct current is normally made to flow through the microphone contacts and this direct current is modified by the microphone contacts, when speech of pitch c0 is uttered in front of the microphone, to convert or modify the direct currentwhich exalternating current having a frequency 00. So the telephone receiver has for its core a permanent magnet or, what amounts to the same thing, an electro magnet which is excited by a direct current. In the system of the present application it is to be understood that a single phase alternating current of a definite frequency replaces the direct cur rent which excites the microphone and. that a single phase alternating current, which is also of a definite frequency, replaces the direct current which excites the telephonereceiver.

Principle oft-71c imwniica-l now propose to demonstrate the principles upon which my invention is based and which may be stated as follows. lfa telephone receiver, instead of being excited by direct currents, is excited by single phase alternating currents of frequency (1' and if it is called on to reproduce sound of pitch 0 uttered in front of a microphone excited, not by direct currents, but by single phase alternating currents of a frequency a the sounds which the telephone receiver will emit will be completely altered and will be inaudible if the difference o, 11 a1 is sulliciently high. If, however, the frequency 0', of the single phase alternating currents which excite the telephone receiver is equal to the frequency 0/, of the single phase alternating currents which excite the microphone and upon which the voice waves of frequency w are impressed, then the telephonereceiver will reproduce these voice waves of frequency a), and it will produce no other sounds if 2.0 is large as compared with (0.

Considering Fig. l, we find a local circuitcontaining a generator A of single phase alternating currents of an electromotive force a, which we will suppose to have a virtual e value of and a frequency 1,. It follows that e=c0 sin 2 M.

This local circuit also contains a microphone c in front of which are supposed to be emitted These sounds of a pitch cc cause electrical waves or vibrations of a frequency 0 to be superimposed upon and to appropriately modify the alternating currents of frequency a with which the microphone has just been assumed to be excited. Thatis to say, instead of exciting my microphone by a source of direct current, as is the present commercial practice, I have excited it by a single phase alternating current of frequency a, and I have superimposed thereupon electrical waves of a frequency w caused by sounds of a pitch (0. The local circuit of Fig. 1 finally contains the secondary coil (1 of a telephone receiver B, the priinary coil 1) of which is supposed to be eX- oited by a single phase alternating current of frequency a That is to say, instead of exciting the core of the telephone receiver by means of a direct current flowing in a coil placed on the core which is the equivalent of the commercial practice of to-day, I excite this telephone receiver by single phase alternating currents of a frequency 01 as has just been stated. These currents are assumed to a have a virtual value of and their intensity 2 l n 7 18 therefore given by the equation j a sin 2 2t.

Let us now assume that the resistance of this local circuit is high as compared with its self inductance, so that we may consider the impedance as equal to the resistance. The intensity 2' of the current flowing in this local circuit will then be expressed by Let us furthermore designate by M a constant relating to the coil b of the telephone receiver B and by N a similar constant relating to its coil 0. The effort F impressed on the telephone plate may then be represented 9 +N 6o sin? 2 ltlyi+yz Sill 2 231 +MNaco [cos 2 1- 2)tc0s 2 1+ 2)tl [yr-He sin 2 6].

I arrange matters so that the magnetization produced by the coil 1), which replaces the permanent magnetization of the ordinary telephone receiver, is very high as compared with the magnetization due to the current from the microphone flowing through the coil a. The constant M will then be very large as compared with the constant N. Since then the factor b is very small as compared with the factors M and MN, we may consider the product represented in the second row of the above expression for the eflort F to be negligible as compared with the other products in this expression. We have, then, to determine what physical meaning is to be given to the terms contained in the first and third rows of the above expression J for the effort F exerted on the receiving telephone plate. The quantity contained in the first row of the above expression for the frequency 0',.+ 0' It further contains the products j zh'lNaeu sin 2 t [cos 2 'i g)tcos 2 1+ 2)5] which, disregarding the constant multiplying factor MNac may be written as the sums of the sines of terms of variable frequencies 0x uq-lco; 0, 11 (0; a, 1i' c0; a +a, m. That is to say if we disregard the term given in the second row of the expression for the effort F which is impressed on the telephone receiver plate, this expression for the effort F will include constant terms and constant multiples of sines or cosines of seven frequencies 211 u, (r (rd-a a, a c0; a a (0; (1 a'.,+ cc; a +(r on. It will not, in general, contain a mere con stant multiple of the sine or cosine of a frequency c0 which will determine the reproduction of speech. Speech, in the general case when a, is not equal to (1,, will therefore not be produced. 011 the other hand the telephone receiver will emit sounds the lowest pitch of which is manifestly represented by the frequency :Y,-a w which is the smallest frequency of the seven just specified. If this frequency a,-a',c0 is taken sufiiciently high, the lowest pitch emitted by the telephone receiver will be above audible limits, which will mean that the telephone receiver will emit no audible sound. Let us now make a,= 11,; that is to say, let us make the frequency of the single phase alternating current which energizes the microphone c and upon which the voice waves of frequency co are impressed, and which current is then sent through the coil (1, equal to the frequency of the single phase alternating current .which passes through the coil 1) to magnetize the receiving telephone plate. Under such assumption, the first row in the expression for the effort F is equal to a constant -minus a constant multiple of the cosine of a frequency 2a,. Under such assumption, also, the third row in the expression for the effort F becomes equal to or what is the same thing The term MNaein z sin 2 25 being of the frequency on, will manifestly determine the reproduction of speech by the telephone. The other terms are either constants or they are sine or cosine functions of frequencies the lowest of which is 211', m. It follows, therefore, under the assum tion cr,=a,, that the telephone receiver wil reproduce voice currents of a pitch co; and that it will have su erimposed thereupon sounds the lowest pitch of which is 2a, co. If then'the frequency 2a, is sufficiently high as compared with the frequency w, it will be an easy matter either to have the telephone plate with too high an inertia to give an appreciable intensity to these sounds or to have these sounds of such high pitch as to make them inaudible.

Summarized, what has been said above amounts to this: that if the frequency a, of the single phase alternating currents upon which the voice waves of frequency m are impressed difi'ers from the frequency a, of the single phase alternating currents which excite the telephone receiver, the expression for the effort 1* upon the receiver telephone plate contains no term which is a constant multiple of the sine or cosine of 2am, that is no term able to reproduce speech. If the difference a,a' is great as compared with the frequency w, the telephone will remain completely silent. On the other hand, if the frequency a, of the single phase alternating currents upon which voice waves of a pitch on are impressed is equal to the frequency a, of the single phase alternating currents which excite the telephone receiver, then speech of pitch 00 will be reproduced by the telephone receiver and, in case 2a, is large compared to a), no other disturbing sound will be produced. But this is what I undertook to prove. H

The multiplex telephone system of F 2? 2. In order to utilize the principles which I have just demonstrated in a system of multiplex telephony, I may arrange my telephonic apparatuses and circuits in accordance with the diagram indicated in Fig. 2.

I have here shown a single main line XY which runs between the stations and a return circuit R which may be a wire conductor or the ground. Between the main and the resin (2 1+ )-sin (g -(0)] turn circuits, I have shown six stations, that is to say, three pairs of stations. A description of one of these stations will sufiice for all.

Taking the station toward the upper left hand corner of Fig. 2, it is seen to comprise an alternator A, which feeds the circuit containing the coil a, surrounding the core of a telephone receiver B, and a microphone c, whichis arranged in shunt of one of the windings e, of an induction coil 1),. In a branch between the main line KY and the return circuit R are arranged, in series, a winding b about the core of the telephone receiver B, and the other winding (Z, of the induction coil 1),. Similar remarks apply to the stations at which are found the alternators A A31 l z: and

We assume that the pair of alternators A, and A, both generate single phase alternating currents of frequency a, and that these currents are of the same phase. We furthermore assume that the pair of alternators A, and A, generate alternating currents of frequency a, of the same phase and that the pair of alternators A, and A, generate alternating currents of the same phase and of the frequency a When speech of pitch to is uttered in front of the microphone c,, it is manifest that the electrical waves of frequency 0/, generated by the alternator A, will be modified by or have superimposed thereon electrical waves of frequency co and that the induction coil D,will cause these modified waves of a fundamental periodicity a, to pass out over the line KY and to pass into the several branches between the main line and the return, among them the V The . modified alternating current of a fundamental periodicity 0/, which passes through the coil b, on the telephone receiver B,, may be considered as passing through a coil on a telephone receiver which is energized by alternating currents of frequency a, passing through the coil a, and generated by the alternator A,. It follows immediately, from the principles which I have demonstrated, that the telephone receiver B, will reproduce the sounds of periodicity 00 which were uttered in front of the microphone 0,. To avoid confusion, however, it must be remembered that the coil 1), in Fig. 2 now plays the part of the coil a in Fig. 1, that is to say, it plays the part of supplying the voice currents; and that the coil a" of Fig. 2 plays the part of the coil 1) in Fig. 1, that is to say, it supplies the currents for energizing the telephone receiver. Now, in the demonstration of the principles on which this invention was based, I assumed that the magnetizationproduced by the alternating current which normally excites the re ceiver is very large compared to the magnetization produced by the modified voice currents. This means that in Fig. 2 I must arrange matters so that the magnetization produced by the coil a, shall be large compared' to the magnetization produced by the coil b',. Similar remarks apply to .all the other coils a, the magnetizing effect of which on the receiver telephones must be large .compared to the magnetizing effect of the coils 1).

Again, in demonstrating the principles on which this invention is based, I assumed that the resistance of the circuit through which the modified voice currents were flowing was high as compared with their self-induction. This means, in Fig. 2, that, the resistance of the circuits containing the coils I), and d, must be made large in comparison with the self induction of these circuits. Similar remarks apply to all the other branch circuits bd.

If matters are arranged as the last two paragraphs require, it willatonce follow from the principles which I have demonstrated that speech uttered in front of the microphone .0, will be reproduced by the telephone receiver B, and, by parityof reasoning, that sounds uttered in front of the microphone 0', will be reproduced by the telephone 13,. It is however necessary that the sounds which are uttered in front of the microphones c, and c,

and which are therefore, in effect, impressed upon alternating currents of frequency a,, should not be heard in any other telephone receivers than the two .telephonereceivers B, and B, just described. To this end I .take the frequencies 0/,, a a assumed to be arranged in ascending order, so that these frequencies and their differences a 0z,, aq a, are great as compared with the pitch to of the sounds making up speech.- It immediately follows from this that, when speech is uttered in front of the microphone 0,, although the alternating currents of frequency a, will have been suitably modified to reproduce speech provided they are received by telephone receivers energized by alternating currents of frequency (1,, they will not reproduce speech in any one of the four telephone B,, B,, B B which receivers are energized by alternating currents of periodicities a, and a, different from the frequency 03. Therefore, although the voice modified currents of fundamental frequency u,, produced through the intervention of the microphones 0,, and 0, .pass in part through the four telephone receivers 13,, B,, B B,, they will not cause the reproduction of the voice in such receivers. Again, since the frequencies a,, 0' a, and their differences a,a,, a -a are greatas compared withrthe pitch of the sound making up speech, it is clear that the speech which is uttered in front of the microphone c, and reproduced in the telephone receiver B, will not have superimposed upon it any audible extraneous or disturbing sounds.

Summarized, what has ,been said amounts to this: that the station containing .the microphone c, and the receiving telephone B and the station containing the microphone c, and the receiving telephone B, can intercommunicate telephonically precisely as if no other stations were present, and this by the use of a single main line and return circuit and single phase alternating currents. Since similar remarks apply to the other pairs of stations which are branchesfrom the same main line, it will be seen that the problem of multiplex telephony has been solved in the manner indicated at the outset of this specification.

The multiplex telephone system of Fig. 3.In the multiplex telephone system which has just been described, it has been assumed that the respective pairs of alternators produce single phase currents in phase with each other. This result may be obtained in a variety of ways; but in order to avoid the complications which would thus be introduced and to have a central source of alternating current supply, I have devised. the multiplex telephone system which is shown in Fig. 3. Here we have the same main line XY and the return circuit R, and three alternators A,, A A located at some convenient point and branched between the main line and the return circuit. The alternator A, generates single phase alternating currents of frequency a, the alternator A generates single phase alternating currents of frequency 01 and the alternator A generates single phase alternating currents of a frequency a The different telephone stations have their electrical circuits branched between the main line and the return circuit precisely as was described in the case of Fig. 2. But I insert in series with the branch circuit containing the coila, and microphone 0,, a resonator E, composed of a condenser and a reactance coil so proportioned that this resonator will permit currents of frequency a, to pass therethrough and will block currents of all other frequencies. Since the principles of electrical resonance by which this result may be obtained are now well understood, they need not be enlarged upon here. I also insert in the branch a c an electrical resonator tuned to the frequency a,. So I insert in the branches a, a and a c electrical resonators E and E each tuned to afrequency a, and I also insert in the circuits a, c.,, and 0/ 0, electrical resonators E and E each tuned to the frequency 0 I finally insert in the alternator branches A,, A A electrical resonators E,, E,, and E respectively tuned to the frequencies a,, 01 ,0

In order that the electrical resonators may act most efficiently and may discriminate most effectively between the different periodicities a,, a 01,, I must make the self-induction of what I may call the a c circuits large and their resistances small. In order that but little of the alternating currents generated by the alternators A, A A shall be diverted into what I maycall the b d circuits, I make the resistances of these I) d circuits large and their self induction small.

I here point out that by making the selfinduction of the a c circuits large and their resistance small and by making the resistance of the b d circuits large and their self-induction small, for the purpose of efficiently distributing to the desired branch circuits and to no others the alternating currents gen erated by the .alternators A,, A A I have made the relation of self-induction to resistance in the a c and b d circuits precisely as was required and done in the system of Fig. 2 for the puspose of having a multiplex telephone system operating in accordance with the principles laid down at the beginning of this specification.

Matters having been arranged in Fig. 3, as thus described, it will be. seen that alternating currents of a frequency a, pass into the two branches (1, c, and a, c, and into no other branch circuits, at least in any apprecia ble quantity. It follows from all this and from what was said in the last aragraph that sounds uttered in front of Hie mic-rophone 0, will be reproduced by the telephone receiver B,, and that sounds uttered in front of the microphone 0, will be reproduced by the telephone 13,, precisely as was described in the case of Fig. 2. But when sounds are uttered in front of the microphone c, and modified alternating currents of frequency are sent out over the line XY, these modified alternatin currents will not only pass into the branc circuits b, d,, where they will reproduce speech in the tele hone receiver B, which is energized by an a ternating current of frequency L1,, but they will also pass into the other I) d circuits because these I) d circuits are not, so far as has been described, electrically tuned. It is true that the passage of the modified alternating currents of frequency a,, which are sent out from the microphone 0,, into the other I) d circuits will not cause the reproduction of speech in the telephone receivers in such circuits, since these receivers are energized by currents of different periodicities a, and u But such diversion of modified alternating current of frequency a, to the other I) d branches will lessen the strength of such current which passes into its appropriate receiving branch tliis butting down of the effective strength of the modified alternating current Wl'llCll asses into its appropriate receiving branch, Iinsert resonators in the b d circuits as fol lows:

I insert resonators F, and F, electrically tuned to the frequency a, in the branches b, d, and b, d, respectively. I insert resonators F and F 2 each electrically tuned to the fre uency 01 in the .branch circuits 1), d, and b d, respectively. And I insert resonators Merely for the purpose of preventing quency a, in the branches 1), (Z and b d resingle phase alternating currents of different F and F 3 each electrically tuned to a fre spectively. In this manner I effect the following results. When speech is uttered'in front of the microphone a, and modified alternating currents of frequency a, are sent out over the line, these modified alternating currents will not be diverted into the branches b 1 b (1,;5, d,; 1), (1' since these branches are now tuned respectively to frequencies a, and 01 and not to the frequency a,. Therefore, the modified currents ofv frequency sent out over the line will pass, with practically undiminished strength, into the appro priate branch I), d, to there reproduce speech.

I may remark, by way of conclusion, that I am not restricted to magneto telephones for my telephone receivers but may employ a condenser as a receiver. So too I may use other sources of alternating currents than a rotary alternator For convenience of phraseology, I refer to the three or more alternating currents of frequencies 01,, a a as a set of alternating currents and I refer to the sound waves which are supposed to be produced in front of the microphone 0,, c c assumed to be the sendin stations, as a set of sound waves. It wi 1 thus be seen that in my method of multiplex telephony, I modify, respectively, in accordance with sets of sound waves, sets of frequencies.

I claim:

1. The method of transmitting sounds telephonically which consists in modifying, in accordance with sound waves, single phase alternating currents of a given frequency, combinin at a distant point the currents thus produced with alternating currents of the same frequency, and translating the resultant currents into sound waves, substantially as described.

2. The method of transmitting sounds tele phonically which consists in modifying, in accordance with sound waves, single phase alternating currents of a frequency which is high compared to the pitch of the sound, combining at a distant point the currents thus produced with alternating currents of the same frequency, and translating the resultant currents into sound waves, substantially as described. i

3. The method of multiplex telephony F which consists in modifying respectively. in

accordance with sets of sound waves, sets of single phase alternating currents of different frequencies, combining at a distant point .each modified current thus produced with an alternating current of its same frequency, and translating the resultant sets of currents into sets of sound waves, substantially as described.

4. The method-of transmitting sounds tele phonically which consists in modifying, in accordance with sound waves, single phase alternating currents of a given frequency, and sending the modified currents through a telephone receiver excited with alternating currents of the same frequency, to thus trans late the modified currents into sound waves, substantially as described.- 5. The method of multiplex telephony which consists in modifying, respectively in accordance with sets of sound waves, sets of single phase alternatin currents of different frequencies, and trans ating each modified alternating current into sound waves by sending it-through a telephone receiver excited by an alternating current of its same frequency, substantially as described.

6. The method of multiplex telephony which consists in modifying respectively, in accordance with sets of, sound waves, sets of single phase alternating currents the frequencies of which are high and the differences of whose frequencies are high compared with the pitch of the sound waves, combining at a distant point each modified current thus produced with an alternating current of its same frequency, and translating the respective resultant sets of currents lnto sets of sound waves, substantially as described. a

7. The method of multiplex telephony 

